Good PS2 games that allow you to explore vast areas?

My young sons have quite a naff game called Smugglers Run, but they love it.What they love about the game is that you can drive for ages over the terrain and find new places, in other games you seem to get to the end of the terrain very quickly. Can anyone recommend any good PS2 games where you can drive or fly (and land) for vast distances and discover new places. To the witless pricks who are no doubt going to reply with ' why don't you take your kids out exploring the real world and get some exercise?' etc, they do get out every day and play in a local forest...but they also love their playstation...it's not a crime is it??